Prior art systems use a needle that extends beyond the distal end of a cannula. The needle includes multiple markings along its length that extends beyond the cannula. The markings are counted after the needle is inserted. The number of markings is then translated into an insertion depth using a look-up table.
Other prior art needles and cannulas use markings at their tips that are detectable with X-rays and/or MRI systems to show the location of the tips with respect to the surrounding tissue.